The extracellular hemoglobin of annelids and tube worms are multisubunit proteins of up to 200 polypeptides and molecular masses to at least 3,900 kDa. They differ from all other Hbs in having 02 binding chains and "linker" chains. The linker chain LI of the hemoglobin of Lumbricus terrestris contains a 38-39 residue segment with a repeating pattern of cysteinyl residues: (CYSX6)3 -CysX5CysX I O-Cys. This pattern, not present in any globin sequence, corresponds exactly to the cystein rich repeats of the ligand binding domain of the low density lipoprotein (LDL) receptor of man and Xenopus laevis. The disulfide connectivity of these domains has not been determined in the LDL receptor. The determination of the disulfide bonds in the linker chains is therefore an important first step in the understanding of the interaction between apolipoprotein E and the LDL receptor. Linker chain L I was digested with a number of enzymes and theresulting peptide mixtures were analysed by matrix-assisted laser desorption mass spectrometry. Due to the huge number of possible disulfide bridges, the assignment of peaks to disulfide bridges containing peptides was extremely difficult. Therefore HPLC fractionation of peptide mixtures were carried out. For the identification of disulfide containing peptides fractions of interest further enzymatic and chemical reactions are performed. Identification of the resulting peptide products provide information that is useful for mapping the disulfide linkages. Ultimately, we determined that MALDI-ITMS of the enzyme digested protein provided the highest quality mapping data. A paper describing the technique and discussing the results is in preparation.